Springfield Plateau Chapter of the Missouri Master Naturalist™ is a community based natural resource education and volunteer program. Its purpose is to develop a corps of well-informed volunteers to provide education, outreach and service dedicated to the beneficial management of natural resources and natural areas within their communities for the State of Missouri.

Saturday, January 14, 2017

The Case of the Case-carrying Worm

Dero (Aulophorus) vaga - 1mm - Linda Bower

I
was following correspondence from Linda Bower asking Chris Barnhart for
identification help after filming an eccentric worm that appeared to be
dragging around a decorated case. Chris recognized that the case was
partly made of Bryozoan statoblasts (the oval brown objects with a pale
perimeter). Chris Barkau, Graduate Research Assistant at Southern Illinois University Carbondale was able to identify it as Tubificida: Naididae: Naidinae: Dero (Aulophorus) vaga. I asked Linda to describe her find.

Linda:
Case-carrying
Worms dwell in ponds with Duckweed, but are often missed by traditional
collecting methods. They are tiny – less than 1mm. It was difficult to
find recent* detailed information on Dero vaga, also called Aulophorus vaga. We know that it forms protective tubes by means of a viscid secretion from their bodies. You can watch this video of
the worm sliding in and out of its case as it searches for food. It is a
fascinating dance you won't see elsewhere, and it is free!

Cut and whole Bryozoan colonies - Click to enlarge - MDC

The Case-carrying Worm is interesting enough, but combined with Bryozoan statoblasts? Wow! Bryozoans
(aka Moss Animals) are really animals, but given their appearance, that
is hard to believe. Bryozoans are a major animal group, having nearly
4,000 known species and only a few dozen of those live in freshwater
habitats. They may grow on any submerged object, such as rocks, roots,
and branches. They feed on protozoans, bacteria, and organic matter from
the water. They are colonial, living in gelatinous blobs.

Most
freshwater species produce resistant bodies called statoblasts that form
in response to adverse environmental conditions and provide a means of
overwintering. As they grow the statoblasts produce bi-valve shells made
of chitin, the same stuff that makes the exoskeleton of arthropods (think insects and crayfish). More detailed information on Bryozoans is at this link.

I
have filmed several Case-carrying Worms since this first find and here
are three additional videos. Do not resist your temptation – follow
these links:

Editor's note: You can bail out now or follow along for the details of the complicated life of D. vaga (or if you prefer A. vaga). These tiny (1mm) worms can be found floating within a mass of duckweed or clustered in the algae of the pond bottom, moving up or down based on the availability of food. Like all other Oligochaeta (worms) D. vaga is a hermaphrodite. It is capable of sexual reproduction or fission. This was described in an 1899 paper, The Natural History and Morphology of Dera vaga.

The period of sexual
reproduction occurs during the first two weeks of July, when the body cavity
posterior to the clitellum is crowded with eggs.Asexual reproduction by fission takes place
throughout the year, but most rapidly during warm weather, when it may occur as
often as three times a week. Three fission zones have been observed in one
individual at the same time.

Click to enlarge

As the animal grows in
length, the case which it inhabits is extended, and after fission the two
daughter worms divide it by placing their heads together at its middle and
forcibly breaking it, each worm then swimming away with one-half of the old
case. The fission zone is formed near the middle of some segment, usually back
of XVII and in front of XXII. The new head and tail are almost completely
formed before separation takes place. The number of somites in the new head is
constant, being five, while twelve to sixteen segments are visible in the tail
before a second fission begins.

Worms divided by
cutting regenerate the missing part, though only enough segments are
regenerated at the anterior end to complete the cephalized portion, i.e., the
first five. Thus if two are removed but two regenerate, while if seven are
taken away only five new segments are formed. At least three or four segments
in addition to the five in the cephalic region are necessary for the
regeneration of the tail.

* There are detailed descriptions of this Oligochaete available when searching name variations.

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About Us

We are a retired physician and nurse, writing to advance the cause of nature through education. We are active in Missouri Master Naturalists as well as a wide variety of like minded Conservation organizations.